1. Field of the Invention
The present invention relates to an apparatus and method for testing quality of an optical disc, and more particularly, to an apparatus and method for testing the quality of the mechanical endurance of a surface of an optical disc.
2. Discussion of the Related Art
Up to now, there are recording media such as a magnetic recording tape, a laser disc (LD) or a compact disc (CD) as an optical disc, and a digital video disc (DVD) having a vast recording capacity.
Since the optical disc among the recording media utilizes a digital recording system that is different from a conventional recording system used in a magnetic recording system, and has a very small volume and weight so that it is efficient and convenient to keep and carry, it is a recent trend for consumers to prefer the optical disc.
It is also important that any appliance may be used without material defects that effect performance. Problems in quality may adversely effect the manufacturer reputation for reliability.
This presents a more difficult challenge in producing an optical disc having minute signal characteristics and where inferior quality may originate from errors in thickness of a disc, scratches, deformity, fingerprints, and attachment of foreign material during manufacturing of the product.
Thus, a produced optical disc generally undergoes quality testing before being shipped to an optical disc market.
A conventional quality test for an optical disc is generally performed by four measuring drives.
First, under the assumption that characteristics of optical discs manufactured by the same equipment are identical, the manufacturer makes a choice of an arbitrary optical disc from all of the manufactured optical discs and loads the selected optical disc onto a measuring system.
In the first measuring drive, a high frequency wave signal and a jitter are measured by means of a signal reproduced from the optical disc.
In the second measuring drive, a servo signal (a focusing error signal and a tracking error signal) are measured based on a signal reproduced from the optical disc.
In the third measuring drive, the mechanical characteristics of the optical disc, which undergoes the quality test, are measured.
Finally, in the fourth measuring drive, the optical characteristics of the optical disc are measured.
As described above, according to the conventional test, the accuracy of the information recorded on the optical disc, and the mechanical characteristics and an optical characteristics of the optical disc are inspected.
Mechanical damage that occurs on an incident surface of the optical disc may be determined by application of an information recording/reproducing laser beam during the use of an high density optical disc. A scratch or any defect may cause the deterioration of a signal an optical disc as well as data loss. In more severe cases, the defect may make the recording and/or reproducing of information onto the optical disc impossible. Thus, detecting such damage takes the most important portion of the quality test of the optical disc.
Therefore, to reduce these types of defects, a protective coating may be formed to enhance the mechanical stiffness or hardness of the surface of the optical disc.
However, after forming the protective coating on the disc surface for this purpose, the mechanical characteristics of the protective coating should be determined. Namely, there is need to determine for how long the protective coating may resist scratches that would occur during the use of the optical disc.
There are a pencil hardness test and a taber abrasion test as a test for the quantization of the high-density optical disc.
The pencil hardness test is a method for estimating a scratch at a hardness value corresponding to the hardness of a pencil at an instant when the scratch occurs by applying workpieces of different hardness to the optical disc in a rectilinear motion.
However, the pencil hardness test has disadvantages in that it is difficult not only to maintain the same load continuously, but also to generate the quantization scratch because the scratch does not occur as frequently as desired.
The taber abrasion test is a test to estimate the degree of endurance by uniformly wearing the surface of the optical disc while applying a predetermined load by means of the D1004 method under the standard, American Society for Testing and Materials (ASTM) standard using a kind of an abrasion wheel.
In the conventional taber abrasion testing apparatus, if an optical disc to be scratched is loaded and rotated, a plurality of abrasion wheels having a predetermined weight contact the optical disc at a perpendicular position to the optical disc to scratch the surface of the optical disc under a uniform load for at least more than ten rotations of the optical disc.
The ASTM defines the load generated from the abrasion wheel to be less than 9.8 N (1000 gf), and there are several kinds of abradants such as CS-10F, CS-10, and CS-17 that may be used.
The taber abrasion testing apparatus as described above is an equipment designed for the purpose of not testing the mechanical endurance of the surface of an optical disc by scratching the optical disc, but testing the degree of the scratch of general plastic goods.
Therefore, the scratch produced by the taber abrasion testing apparatus is remarkably different from the scratch generated when an optical disc is used actually in real life.
Namely, the taber abrasion testing apparatus determines the endurance of the surface of the optical disc by wearing the surface. This is remarkably different from the situation that real-generated scratch is reacted in a drive (for example, an optical disc drive) for driving the optical disc by responding the operation.
Therefore, since a manufacturer must determine the quality of the optical disc by his subjective determination based principally on experience and know-how alone, not quantitative classification according to an actual reference standard, errors frequently occur. Moreover, since the shape of the scratches generated on the optical disc in real life when compared with those generated by the taber abrasion testing apparatus are different, attempts to determine whether an optical disc is poor or good with only the know-how without absolute reference value have resulted in a vast number of errors.
Thus, an urgent demand exists for a special method for quantizing the degree scratch generation in order to test the mechanical endurance of surface of an optical disc.